1. Field of the Invention
The present invention relates generally to optical systems for use with visible and near-visible wavelengths of light. More particularly, the present invention relates to such optical systems, used in optical data storage devices.
2. Related Art
Optical systems use a variety of lenses and other elements to manipulate light. The basic element used to focus a light beam in a simple optical system is referred to hereinafter as an objective lens. A simple optical system is illustrated in FIG. 1, in which a light beam 101 passes through a first medium 103, typically air, through a first interface 105 into a second medium 107 of which the objective lens 108 is formed, and then through a second interface 109 back into the first medium 103. The objective lens 108 focuses, or causes to converge, rays of light 111, 113 comprising the light beam 101, so that they form a small spot 115 at a target 117.
The simple optical system of FIG. 1 has an apparent resolving power which depends upon the effective numerical aperture achieved by the system design. Numerical aperture (NA) is a parameter known to those skilled in this art and readily calculated based upon the wavelength of the light involved, the indices of refraction (n) of the first and second media, and other readily measured physical characteristics of the system.
Immersion lenses of various types are well-known in the field of optical microscopy to improve the apparent resolving power of optical microscopes. Common immersion lenses used in optical microscopy include oil immersion lenses and solid immersion lenses (FIG. 2, 201), both of which are located between the objective lens and the target, thereby increasing the numerical aperture of the system. Achievable numerical aperture figures include NA&gt;1. Immersion lenses, including solid immersion lenses achieve the high NA figures by conducting light to the target through a medium having a high index of refraction which is either in contact with or evanescent wave coupled to the target. Evanescent wave coupling occurs when the wavefront of a light beam extends a microscopic distance past an interface surface before being internally reflected at the interface surface. Such a wave can couple into adjacent materials, such as the target.
The use of solid immersion lenses has been extended to the field of optical data recording, for example as disclosed by Corle et al. in U.S. Pat. No. 5,125,750, incorporated herein by reference in its entirety. The system disclosed by Corle et al. includes a SIL having a flat lower surface adjacent the target and a hemispherical upper surface facing an objective lens. A laser light beam is shone through the objective lens and the SIL onto the target. Corle et al. teach that the hemispherical shape of the top surface of the SIL ensures that light rays comprising the light beam which has been focused by the objective lens enter the SIL normal to the hemispherical top surface of the SIL, while the flat shape of the lower surface ensures that the SIL can be located at a uniform distance from the target, i.e., a data recording medium. Corle et al. teach that use of their system reduces the spot size of the light focussed on the target by a factor 1/n, where n is the index of refraction of the SIL material.
The use of SILs in optical data recording has been advanced beyond the teachings of Corle et al. noted above, for example by Mamin et al., as disclosed in U.S. Pat. No. 5,497,359, incorporated herein by reference in its entirety. Mamin et al. teach that the hemispherical SIL of Corle et al. can be improved upon by increasing the thickness thereof at the center beyond the thickness a hemispherical shape would have. Marnin et al. reduce the spot size of the light focussed on the target by a factor 1/n.sup.2, where n is again the index of refraction of the SIL material.
The use in an optical disk flying head of a SIL with a hemispherical top and a curved bottom surface is disclosed by Berg et al. in U.S. Pat. No. 5,729,393, incorporated herein by reference in its entirety. The lower surface of the SIL disclosed in Berg et al. has a curve which stabilizes the flight and performance of the device in a flying optical disk drive head. The radius of curvature used is not more than about 10 meters.
SILs as known in the art present certain problems, which make their use difficult and expensive in practical optical disk drive storage systems. In combination with a flat bottom surface, the hemispherical shape or other bulging shape is difficult to construct, difficult to align with other components of an optical system and imposes spacing and tooling constraints on the optical system design.